Capacitor and semiconductor device having a ferroelectric material

ABSTRACT

A capacitor includes a pair of electrodes and a ferroelectric film sandwiched between the electrodes. The electrodes are provided perpendicular to the direction of the polarization axis of the ferroelectric film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application U.S. application Ser.No. 11/121,054, filed on May 4, 2005, which is a division of U.S.application Ser. No. 10/718,726 filed on Nov. 24, 2003 now U.S. Pat. No.6,943,080, issued on Sep. 13, 2005. The present application is based onJapanese Laid-Open Patent Application No. 2002-358093, filed on Dec. 10,2002, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a capacitor, a semiconductordevice including the capacitor, and a method of manufacturing thesemiconductor device, and more particularly to a nonvolatilesemiconductor storage device including a capacitor using a ferroelectricmaterial as a dielectric.

2. Description of the Related Art

Some semiconductor devices, particularly, semiconductor memories, have avariety of properties. The semiconductor memories are roughly classifiedinto volatile memories that lose information stored therein when poweris turned off and nonvolatile memories that retain information storedtherein when power is turned off. A nonvolatile memory that retains datain a capacitor using a ferroelectric material as a dielectric isreferred to as a ferroelectric random access memory (FRAM®).

The FRAM uses the two remanent polarization properties of differentpolarities of a ferroelectric material so as to retain data when poweris turned off and no electric field is applied across the capacitor. TheFRAM can rewrite data 10¹⁸˜10¹² times, which is far more than the numberof times (10⁶) a flash memory can rewrite data. The FRAM can rewritedata at high speed in tens of nanoseconds.

In the FRAM, the ferroelectric material, which is material for thedielectric of the capacitor, is polarized in one of two directions uponapplication of an electric field, and remains polarized in the samedirection even after removing the electric field. Data of logical “1”and “0” corresponding to one and the other, respectively, of thepolarization directions can be recorded in the FRAM by distinguishingbetween the polarization directions. Further, the polarization directioncan be switched from one to the other by applying a sufficient electricfield in the direction opposite to the polarization direction.

Typical ferroelectrics include lead-based ferroelectrics such asPbZr_(1-x)Ti_(x)O₃ (0≦x≦1) (PZT) and Pb_(1-y)La_(y)Zr_(1-x)Ti_(x)O₃(0≦x, y≦1) (PLZT) and bismuth-based ferroelectrics such as SrBiTa₂O₉(SBT)

Generally, in PZT, remanent polarization is reduced as the reversal ofpolarization is repeated, causing property degradation (fatiguephenomenon). Meanwhile, the FRAM, which uses one of such ferroelectrics,is employed in apparatuses handling personal or money information, suchas IC cards. Accordingly, the FRAM is required to be highly reliable. Inorder to realize the expected durable period of ten years, it isdesirable that the FRAM have a larger remanent polarization charge.

However, most conventional ferroelectric capacitors manufactured byspattering or the sol-gel method have a remanent polarization charge of20-25 μC/cm². Only tens of percents of the conventional ferroelectriccapacitors satisfy a remanent polarization charge of 30 μC/cm² requiredfor a product. FIG. 1 is a diagram showing a conventional ferroelectriccapacitor 100 of the FRAM Referring to FIG. 1, the ferroelectriccapacitor 100 is formed by successively layering a Pt film 101 as alower electrode, a PZT film 102 that is a ferroelectric film, and a Ptfilm 103 as an upper electrode in the order described on, for instance,a contact plug connected to a diffusion region of a transistor. Thelower-electrode Pt film 101 is oriented in the [111] direction towardthe direction of film thickness by its self-orientation characteristic.The PZT film 102 formed on the Pt film 101 is oriented in the [111]direction, affected by the orientation of the Pt film 101. Since the PZTfilm 102 has a polarization axis in the [001] direction, the directionof the electric field applied between the lower-electrode Pt film 101and the upper-electrode Pt film 103 differs from the direction of thepolarization axis of the PZT film 102. This results in the problem thatan electric charge usable as remanent polarization decreases naturally.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea capacitor in which the above-described disadvantage is eliminated.

A more specific object of the present invention is to provide acapacitor that can acquire a greater remanent polarization charge, asemiconductor device including the same, and a method of manufacturingsuch a semiconductor device.

One or more of the above objects of the present invention are achievedby a capacitor including a pair of electrodes and a ferroelectric filmsandwiched between the electrodes, wherein the electrodes are providedperpendicular to a direction of a polarization axis of the ferroelectricfilm.

According to the above-described capacitor, the electrodes are providedperpendicular to the direction of the polarization axis of theferroelectric film Accordingly, the charge of the maximum remanentpolarization of the ferroelectric film may be obtained.

One or more of the above objects of the present invention are achievedby a semiconductor device including: a semiconductor substrate; and acapacitor provided on the semiconductor substrate, the capacitorincluding a pair of electrodes and a ferroelectric film sandwichedtherebetween, wherein the electrodes are provided perpendicular to adirection of a polarization axis of the ferroelectric film.

One or more of the above objects of the present invention are alsoachieved by a semiconductor device including: a semiconductor substrate;a transistor formed on the semiconductor substrate, the transistorincluding a gate electrode and a diffusion region; a first interlayerinsulating film covering the semiconductor substrate and the transistor;a second interlayer insulating film formed on the first interlayerinsulating film; and a capacitor formed in the second interlayerinsulating film, the capacitor including a pair of electrodes and aferroelectric film sandwiched therebetween, wherein the electrodes areprovided perpendicular to a direction of a polarization axis of theferroelectric film.

According to the above-described semiconductor devices, the electrodesare provided perpendicular to the direction of the polarization axis ofthe ferroelectric film in the capacitor. Accordingly, the maximumremanent polarization of the ferroelectric film can be extracted.Therefore, the above-described semiconductor devices have tolerance tothe degradation of a remanent polarization charge and have a highsignal-to-noise ratio at the time of reading out information.Accordingly, the reliability of the semiconductor devices can beincreased.

The above objects of the present invention are further achieved by amethod of manufacturing a semiconductor device including a capacitorincluding a pair of electrodes and a ferroelectric film withferroelectricity sandwiched therebetween, the method including the stepsof: (a) depositing the ferroelectric film on a first substrate; (b)forming the capacitor by grinding the ferroelectric film and forming theelectrodes so that the electrodes are perpendicular to a direction of apolarization axis of the ferroelectric film; (c) forming a firstinterlayer insulating film covering a surface of the first substrate andthe capacitor; (d) forming a transistor on a second substrate, thetransistor including a gate electrode and a diffusion region; (e)forming a second interlayer insulating film covering a surface of thesecond substrate and the transistor; (f) flattening surfaces of thefirst and second interlayer insulating films by chemical mechanicalpolishing; (g) integrating the first and second substrates by joiningthe flattened surfaces of the first and second interlayer insulatingfilms; and (h) removing the first substrate.

According to the above-described method, since the ferroelectric film ofthe capacitor is formed on the single-crystal substrate, theferroelectric film may be formed epitaxially thereon. Therefore, theferroelectric film can be formed with excellent crystalline quality witha fixed crystal orientation. Further, in the semiconductor device, thecapacitor is formed in a process independent of the process of formingthe transistor. Accordingly, a wide range of tolerance can be set forthe conditions for forming the capacitor, such as a temperaturecondition, so that the ferroelectric film may be formed with bettercrystallinity. As a result, the reliability of the semiconductor devicecan be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing a conventional ferroelectric capacitor of aFRAM;

FIG. 2 is a diagram showing a perovskite structure;

FIG. 3 is a diagram for illustrating the principles of the presentinvention;

FIG. 4 is a diagram showing an equivalent circuit of a FRAM according toan embodiment of the present invention;

FIG. 5 is a sectional view of the FRAM according to the embodiment ofthe present invention;

FIGS. 6A through 6K are diagrams showing a process of manufacturing theFRAM according to the embodiment of the present invention;

FIG. 7 is a sectional view of a layered structure in which aferroelectric film is formed according to a first variation of theembodiment of the present invention;

FIG. 8 is a sectional view of a layered structure in which theferroelectric film is formed according to a second variation of theembodiment of the present invention; and

FIG. 9A is a plan view of a ferroelectric capacitor and FIG. 9B is asectional view of the ferroelectric capacitor of FIG. 9A taken along theline X-X according to a third variation of the embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a brief description is given of the principles of the presentinvention.

In the following, the case of using a perovskite oxide (an oxide havinga perovskite structure) as the ferroelectric film of a ferroelectriccapacitor is described as an example. Generally, a perovskite oxide isexpressed by the general formula ABO₃. FIG. 2 is a diagram showing aperovskite structure. Referring to FIG. 2, in an ideal perovskitestructure, a unit lattice is a cube with A ions being disposed at thevertexes, a B ion at the body center, and C ions at the face centers. Inthis structure, the large-size A and O ions form a cubic closest packingstructure with the small-size B ion being in the space formed therein.If the oxide is PZT, the A ions are Pb²⁺ and the B ions are Zr⁴⁺ andTi⁴⁺. The spontaneous polarization of a ferroelectric is caused byasymmetrical displacement of the B ions, which are small and mobile, andis reversible by an external electric field. The B ions are mostdisplaceable in the [001] direction. As a result, remanent polarizationis maximized in the [001] direction. Accordingly, when the electrodes ofthe ferroelectric capacitor are disposed perpendicular to the [001]direction of the ferroelectric film, that is, disposed on the (001)plane, the maximum remanent polarization can be extracted as a signalcharge.

FIG. 3 is a diagram for illustrating the principles of the presentinvention. Referring to FIG. 3, a ferroelectric film 11 such as a PZTfilm is epitaxially grown on the (100) surface of, for instance, a MgOsingle-crystal substrate 10 so that the (100) surface of theferroelectric film 11 grows. Accordingly, the ferroelectric film 11 hasthe [001] direction perpendicular to the direction of its growth.Therefore, by exposing two (001) surfaces by etching, and formingelectrodes 12 by depositing conductive material by spattering, aferroelectric capacitor 13 having the electrodes 12 providedperpendicular to the direction of the polarization axis (the [001]direction) is formed. Accordingly, the ferroelectric capacitor 13 canextract a maximum remanent polarization.

Further, according to the present invention, the ferroelectric capacitor13 formed on the single-crystal substrate 10 is disposed on a siliconsubstrate on which a transistor is formed. The ferroelectric capacitor13 is disposed on the transistor substrate by first joining thesubstrate 10 and the transistor substrate together and then removing thesubstrate 10.

It is possible to grow a ferroelectric film such as a PZT filmepitaxially on a silicon transistor substrate. It is impossible,however, to form the ferroelectric film on the surface of the siliconsubstrate or a silicon oxide film (not limited to amorphous material ora polycrystalline substance) formed on the surface of the siliconsubstrate so that the [001] direction of the ferroelectric film isparallel to the main surface of the silicon substrate. That is, it isimpossible to expose (001) surfaces of the ferroelectric film and formelectrodes on the exposed (001) surfaces. Meanwhile, it is possible togrow a buffer layer of, for instance, magnesia spinel, epitaxially onthe silicon substrate, and grow a ferroelectric film on the bufferlayer. However, heating at temperatures as high as 900° C. or over isrequired to form the buffer layer, and the heating causes the problem ofa change in the impurity distribution of the diffusion regions of thetransistor.

Therefore, according to the present invention, a ferroelectric capacitoris previously formed on a single-crystal substrate, and theferroelectric capacitor is disposed by joining the single-crystalsubstrate and a silicon substrate that is to serve as a semiconductordevice Next, a description is given, with reference to the accompanyingdrawings, of an embodiment of the present invention.

FIG. 4 is a diagram showing an equivalent circuit of a FRAM 20 accordingto an embodiment of the present invention. Referring to FIG. 4, a memorycell 14 has a so-called 2T2C-type structure using two transfertransistors T₁ and T₂ and two ferroelectric capacitors C₁ and C₂ inorder to record one-bit information. The memory cell operatescomplementarily, recording information “0” and “1” in one and the other,respectively, of the ferroelectric capacitors C₁ and C₂. Specifically,information is written as follows. The transfer transistors T₁ and T₂are turned on by a word line WL, and information “0” or “1” is input toa bit line BL and information “1” or “0” is input to a complementary bitline /BL, so that complementary information is written to theferroelectric capacitors C₁ and C₂. The written information is retainedas the polarization directions of the ferroelectric films of theferroelectric capacitors C₁ and C₂. Information is read out as follows.When the transfer transistors T₁ and T₂ are turned on by the word lineWL, voltages due to the remanent polarization of the ferroelectriccapacitors C₁ and C₂ are applied to the bit line BL and thecomplementary bit line /BL. Then, a sense amplifier 15 detects thedifference in voltage between the bit lines BL and /BL, so that theinformation stored in the ferroelectric capacitors C₁ and C₂ is readout.

FIG. 5 is a sectional view of the FRAM 20 according to the embodiment ofthe present invention. FIG. 5 shows a section of the FRAM 20 includingthe ferroelectric capacitor C₁ and the transfer transistor (hereinafterreferred to simply as a transistor) T₁ along the bit line BL of thememory cell 14 of FIG. 4.

Referring to FIG. 5, the FRAM 20 includes: a semiconductor substrate 41;the transistor T₁ formed of a gate electrode 43 serving as a word line(the word line WL in FIG. 4) and source/drain regions 44 and 45 on thesemiconductor substrate 41; an interlayer insulating film 48 coveringthe semiconductor substrate 41 and the transistor T₁; an interlayerinsulating film 27 formed on the interlayer insulating film 48; theferroelectric capacitor C₁ (referred to by reference numeral 26) formedof electrodes 25A and 25B and a ferroelectric film 22 sandwichedtherebetween in the interlayer insulating film 27; a contact plug 52 anda via plug 32 that connect the source/drain region 44 and the electrode25A; a via plug 61 that connects the electrode 25B and a planar line 68;and a contact plug 53 and via plugs 33, 62, and 69 that connect thesource/drain region 45 and the bit line BL (referred to by referencenumeral 72).

A well-know silicon substrate is employed as the semiconductor substrate41. In the semiconductor substrate 41, an isolation film 42 is formed bySTI (Shallow Trench Isolation) or LOCOS (Local Oxidation of Silicon) sothat a device region 47 is formed and defined. In the device region 47,the source/drain regions 44 and 45 and the gate electrode 43 as a wordline on a gate oxide film 43A are covered with films including asidewall insulating film 43B so that the transistor T₁ is formed. Thegate electrode 43 extends in a direction perpendicular to the section ofthe FRAM 20 shown in FIG. 5.

The electrode 25A of the ferroelectric capacitor 26 is connected to thesource/drain region 44 through the via plug 32 and the contact plug 52.The electrode 25B of the ferroelectric capacitor 26 is connected to theplanar line 68 through the via plug 61. The ferroelectric capacitor 26is disposed so that a direction perpendicular to the surfaces of theelectrodes 25A and 25B which surfaces are in contact with theferroelectric film 22 is parallel to the main surface of thesemiconductor substrate 41. A detailed description is given below of amethod of forming the ferroelectric film 22. The ferroelectric film 22is initially formed on a single-crystal substrate different from thesemiconductor substrate 41 (as shown in FIG. 6A). The single-crystalsubstrate is, for instance, a MgO single-crystal substrate having a(100) main surface, and the ferroelectric film 22 is grown epitaxiallyon the single-crystal substrate.

Further, the electrodes 25A and 25B are disposed perpendicular to thedirection of the polarization axis of the ferroelectric film 22. Forinstance, when the ferroelectric film 22 is a perovskite oxide, theferroelectric film 22 has its polarization axis in the [001] direction.Accordingly, the electrodes 25A and 25B are provided perpendicular tothe [001] direction, that is, the electrodes 25A and 25B are provided on(001) surfaces of the ferroelectric film 22.

A crystal having, for instance, a perovskite structure, a bismuth layerstructure, or a tungsten bronze structure is employable as theferroelectric film 22. Typical crystals with a perovskite structureinclude PZT expressed by the general formula PbZr_(1-x)Ti_(x)O₃ (0≦x≦1),PLZT expressed by the general formula Pb_(1-y)La_(y)Zr_(1-x)Ti_(x)O₃(0≦x, y≦1), and a crystal expressed by the general formulaPb(B′_(1/3)B″_(2/3))_(x)Ti_(y)Zr_(1-x-y)O₃ (0≦x, y≦1, B′: bivalentmetal, B″: pentavalent metal) orPb(B′_(1/2)B″_(1/2))_(x)Ti_(y)Zr_(1-x-y)O₃ (0≦x, y≦1, B′: trivalentmetal, B″: hexavalent metal). The latter two crystals are preferable toPZT because the remanent polarization of each of the latter two crystalsis greater than that of PZT.

Crystals with a bismuth layer structure include SrBiTa₂O₉ (SBT) andBiLaTi₃O₁₂ (BLT). Crystals with a tungsten bronze structure includeBa₂NaNb₃O₁₅ and Ba_(1-x)Sr_(x)Nb₂O₆.

The electrodes 25A and 25B of the ferroelectric capacitor 26 are, forinstance, 200 nm in thickness. The material of the electrodes 25A and25B may be selected from the platinum group elements of Pt, Ru, Rh, Pd,Os, and Ir, Ti, their alloys, and a group of conductive oxides such asIrO₂, RuO₂, SrRuO₃, CaRuO₃, LaRuO₃, La_(x)Sr_(1-x)CoO₃ (0≦x≦1), andLa_(x)Sr_(1-x)MnO₃ (0≦x≦1). The electrodes 25A and 25B may have a layerstructure of two or more of the above-described materials. For instance,Ir/IrO₂ may be employed so that each of the electrodes 25A and 25B has aconductive oxide layer on the ferroelectric film 22 side. The repeatedpolarization reversal of the ferroelectric film 22 may cause a latticedefect such as oxygen deficiency at the interfaces between theferroelectric film 22 and the electrodes 25A and 25B, thus resulting inthe degradation of the remanent polarization of the ferroelectric film22. The formation of a conductive oxide layer between the ferroelectricfilm 22 and each of the electrodes 25A and 25B can control thedegradation of the remanent polarization of the ferroelectric film 22,thus increasing reliability.

Further, according to the present invention, there is no need toepitaxially grow the ferroelectric film 22 on the electrodes 25A and25B. Accordingly, the electrodes 25A and 25B may be a metal such as W,Al, or Cu. Thus, the material of the electrodes 25A and 25B may beselected from a wide range.

Next, a description is given of a method of forming the FRAM 20according to the embodiment of the present invention.

FIGS. 6A through 6K are sectional views for illustrating a method offorming the FRAM 20 according to the embodiment. FIGS. 6A through 6Eshow the process of forming the ferroelectric capacitor 26 on asingle-crystal substrate 21. FIGS. 6F and 6G show the process of formingthe transistor T₁ on the semiconductor substrate 41 of the FRAM 20.FIGS. 6H through 6K show the process of forming the FRAM 20 by joiningthe single-crystal substrate 21 and the semiconductor substrate 41together.

In the process of FIG. 6A, the ferroelectric film 22 such as a PZT filmis epitaxially grown on the (100) surface of the single-crystalsubstrate 21 such as a MgO substrate Specifically, the (100) surface ofMgO is cleaned, and then the PZT film 22 is epitaxially grown on the(100) surface of the MgO substrate 21. The PZT film 22 may be formed bymetalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy(MBE), pulsed laser deposition (PLD), or the sol-gel method. Thefollowing description is given of the case of using the sol-gel method.Approximately 0.3 cm³ of a PZT thin film forming agent (PZT 113/45/55,of a concentration of 15 weight %) to which Pb is excessively added isdropped on the MgO substrate 21, and the MgO substrate 21 is rotated at3000 rpm for 20 seconds. PZT 113/45/55 indicates that the molarconcentration ratio of Pb to Zr to Ti is 113:45:55. Next, the MgOsubstrate 21 on which the PZT thin film forming agent has been appliedis heated on a hot plate at 350° C. for one minute so that the solventof the PZT thin film forming agent is volatilized. Then, the MgOsubstrate 21 is cooled to room temperature. The single-crystal substrate21 is not limited to a MgO substrate, which may be replaced by a SrTiO₃substrate, a sapphire substrate, or a magnesia spinel (MgAl₂O₄)substrate. The crystal surfaces of these single-crystal substrates arenot limited to the above-described (100) surface, but may be the (010)surface. Since the [001] direction is formed parallel to the mainsurface of the single-crystal substrate, the electrodes 25A and 25B maybe formed on (001) surfaces maximizing remanent polarization.

Further, in the process of FIG. 6A, the PZT film 22 is crystallized byRTA (rapid thermal annealing) processing using, for instance, a halogenlamp annealing apparatus. Specifically, the substrate 21 is placed inthe halogen lamp annealing apparatus and is heated at 650° C. for 10minutes with 5 sccm of oxygen gas being supplied, thereby crystallizingthe PZT film 22. The thickness of the crystallized PZT film 22 is set to80 through 200 nm, for instance, 120 nm. If the thickness of the PZTfilm 22 does not reach a desired value by a single application of thePZT thin film forming agent and a single crystallization process, theapplication of the agent and the crystallization process may berepeated. Each application of the agent can grow a PZT film epitaxiallyon the previously formed PZT film with little effect on thecrystallinity of the PZT film 22. As a result of the crystallization ofthe PZT film 22, the crystallographic relationship between the PZT film22 and the MgO single-crystal substrate 21 is that the (100) surface ofthe PZT film 22//the (100) surface of the MgO substrate 21, and thedirection in the film surface is defined as the [001] direction of thePZT film 22//the [001] direction of the MgO substrate 21.

Further, in the case of forming the PZT film 22 by, for instance, MOCVD,Pb material such as Pb(C₁₁H₁₉O₂)₂, Zr material such as Zr(C₁₁H₁₉O₂)₄,and Ti material such as Ti(OiC₃H₇)₂(C₁₁H₁₉O₂)₂ are employable. Thesematerials are solid at room temperature Therefore, the PZT film 22 maybe formed by sublimation by setting heating temperature in accordancewith the composition of the PZT film 22 and using He as a carrier gas.More commonly, the PZT film 22 is formed by dissolving these materialsin an organic solvent such as THF (tetrahydrofuran), transporting thesolution to a heated vaporization chamber by a mass flow controller(MFC), and vaporizing the solution instantaneously.

Further, in the process of FIG. 6A, the surface orientation of the PZTfilm 22 is specified by the crystal orientation of the MgOsingle-crystal substrate 21, and grinding is performed so as to expose(001) surfaces 22-1 of the PZT film 22. The cube-on-cube epitaxialgrowth of the PZT film 22 on the MgO single-crystal substrate 21 causesthe (001) surface of the MgO single-crystal substrate 21 and the (001)surface of the PZT film 22 to be parallel to each other. Patterning isperformed with a resist 23 with reference to the (001) surface of theMgO substrate 21, and the PZT film 22 is ground by wet etching using anitric acid solution and a hydrofluoric acid solution so that the (001)surfaces 22-1 of the PZT film 22 are exposed.

Next, in the process of FIG. 6B, a resist 24 is formed to cover thestructure of FIG. 6A. Openings 24-1 for forming electrodes on the (001)surfaces 22-1 of the PZT film 22 are formed in the resist 24 bypatterning.

Further, in the process of FIG. 6B, the electrodes 25A and 25B areformed by depositing any of the above-described electrode materials byspattering. In this case, Ir is deposited as a film so as to form theelectrodes 25A and 25B. The thickness of the Ir film is set to 200 nm.Thereby, the ferroelectric capacitor 26 composed of the PZT film 22 andthe electrodes 25A and 25B is formed. Before the formation of the Irfilm, a film of IrO₂ may be formed on the (001) surfaces 22-1 of the PZTfilm 22. This prevents the occurrence of a lattice defect such as oxygendeficiency on the surfaces 22-1 of the PZT film 22. As a result, thedegradation of the remanent polarization of the PZT film 22 can beprevented so as to increase the number of times the FRAM 20 can rewritedata.

Next, in the process of FIG. 6C, the resist 24 as well as the Ir film 25formed thereon are lifted off. The interlayer insulating film 27 of asilicon oxide film is formed by CVD using TEOS (Tetraethylorthosilicate)so as to cover the ferroelectric capacitor 26 and the single-crystalsubstrate 21. The interlayer insulating film 27 is not limited to asilicon oxide film of TEOS, but may be a SiOF film or a BPSG film.However, it is preferable to form the interlayer insulating film 27 byCVD using TEOS or TEOS and ozone. In this case, the interlayerinsulating film 27 may be formed at low temperatures with good covering.Therefore, it is possible to provide sufficient covering over theferroelectric capacitor 26 without causing thermal damage thereto.

Further, in the process of FIG. 6C, a stopper film 28 including asilicon nitride film is formed, and by photolithography and etching,openings 28-1 and 28-2 are formed in the stopper film 28 at a positionabove the electrode 25A and a position where the via plug 33 is to beformed.

Further, in the process of FIG. 6C, an interlayer insulating film 29(also shown in FIG. 5) is formed on the surface of the stopper film 28,and a stopper film 30 is formed on the interlayer insulating film 29. Aresist 31 is formed on the stopper film 30, and openings 30-1 and 30-2are formed in the resist 31 and the stopper film 30 by etching such asRIE (reactive ion etching).

Next, in the process of FIG. 6D, the resist 31 of FIG. 6C is removed.With the stopper films 28 and 30 serving as masks, vias 27-1 and 27-2are formed by RIE so as to penetrate the interlayer insulating films 27and 29 so that the electrode 25A and the single-crystal substrate 21 areexposed.

Next, in the process of FIG. 6E, an adhesion film 31 of, for instance,TiN is formed by spattering on the surface of the structure of FIG. 6Dand the inner walls of the vias 27-1 and 27-2. Then, the vias 27-1 and27-2 are filled with a conductive material such as W, Cu, or Al by CVD,spattering, or plating, so that the via plugs 32 and 33 are formed.

Further, in the process of FIG. 6E, using the stopper film 30 shown inFIG. 6D as a polishing stopper, the conductive material is polished byCMP to be flattened so that an upper surface 29A of the interlayerinsulating film 29 and the surface of the via plugs 32 and 33 are in thesame plane. Next, the stopper film 30 is polished with another polishingagent so that the interlayer insulating film 29 is exposed. In terms ofelectrical connection after the joining of substrates, it is desirablethat the surface 29A of the interlayer insulating film 29 be as flat aspossible.

Next, in the process of FIG. 6F, in the silicon substrate 41 to serve asthe main body of the semiconductor device, the isolation film 42 isformed by STI, and the transistor T₁ including the gate electrode 43 asa word line and the source/drain regions 44 and 45 is formed in thedevice region 47, using a well-known method.

Next, in the process of FIG. 6G, a SiN film 46 is formed over thestructure of FIG. 6F, and the interlayer insulating film 48 is formed onthe SiN film 46 so as to cover the entire SiN film 46. The interlayerinsulating film 48 is formed to have a thickness of 500 nm by CVD usingTEOS.

Further, in the process of FIG. 6G, contact holes 49 and 50 are formedin the interlayer insulating film 48 by photolithography and etching sothat the source/drain regions 44 and 45 are exposed.

Further, in the process of FIG. 6G, an adhesion film 51 of, forinstance, TiN is formed on the surfaces of the contact holes 49 and 50and the interlayer insulating film 48 by spattering. Then, the contactholes 49 and 50 are filled with a conductive material such as W, Cu, orAl by CVD, spattering, or plating, so that the contact plugs 52 and 53are formed.

Further, in the process of FIG. 6G, the conductive material is polishedby CMP to be flattened so that an upper surface 48A of the interlayerinsulating film 48 and the surface of the contact plugs 52 and 53 are inthe same plane. In terms of electrical connection after the joining ofsubstrates, it is desirable that the surface 48A of the interlayerinsulating film 48 be as flat as possible.

Next, in the process of FIG. 6H, the surface 29A of the interlayerinsulating film 29 of a structure 35 of FIG. 6E and the surface 48A ofthe interlayer insulating film 48 of a structure 55 of FIG. 6G arejoined so that the structure 35 of FIG. 6E and the structure 55 of FIG.6G are joined. The structures 35 and 55 may be joined using any oftechniques of joining substrates disclosed in Japanese Patent Nos.2738012 and 2584639, for instance. The single-crystal substrate 21 andthe semiconductor substrate 41 are heated at a temperature in the rangeof 120-450° C. The interlayer insulating films 29 and 48, which areformed of materials of the same type, can be joined easily.

Positioning marks are previously provided to the single-crystalsubstrate 21 and the semiconductor substrate 41 so that the substrates21 and 41 are positioned based on the positioning marks at the time ofthe joining. The substrates 21 and 41 are positioned so that the viaplugs 32 and 33 of the structure 35 of FIG. 6E are connected to thecontact plugs 52 and 53, respectively, of the structure 55 of FIG. 6G.

Next, in the process of FIG. 6I, the MgO single-crystal substrate 21 isremoved so that the ferroelectric capacitor 26 and the via plug 33 areexposed. Specifically, the MgO substrate 21 is dissolved by dilutehydrochloric acid. If the single-crystal substrate 21 is a Si substrate,the substrate 21 can be dissolved in the same manner. If the substrate21 is a sapphire substrate, the substrate 21 may be removed mechanicallyor by CMP.

Further, in the process of FIG. 6I, an interlayer insulating film 56, astopper film 58, and an interlayer insulating film 59 (also shown inFIG. 5) are formed on the exposed surface of the structure of FIG. 6Hafter removing the substrate 21 therefrom. Then, in the same manner asin the processes of FIGS. 6D and 6E, the via plugs 61 and 62 are formedso as to be connected to the electrode 25B and the via plug 33,respectively, with an adhesion film 60 of a TiN film being formed on theboundary between the via plugs 61 and 62 and the interlayer insulatingfilms 56 and 59.

Next, in the process of FIG. 6J, an interlayer insulating film 63 (alsoshown in FIG. 5) is formed on the structure of FIG. 6I. Byphotolithography and etching, a groove 64 for a planar line is formed onthe via plug 61 and a via 65 is formed on the via plug 62 in theinterlayer insulating film 63. A conductive material such as W isprovided over the interlayer insulating film 63, filling the groove 64and the via 65. The conductive material is polished by CMP so that theinterlayer insulating film 63 is exposed. As a result, the via plugs 68and 69 are formed.

Next, in the process of FIG. 6K, the interlayer insulating film 70 isformed on the structure of FIG. 6J. The bit line 72 is formed by thedamascene method so as to be connected to the source/drain region 45through the via plugs 69, 62, and 33 and the contact plug 53. Further, aprotection film 74 is formed on the surfaces of the bit line 72 and theinterlayer insulating film 70. Thereby, the FRAM 20 of FIG. 5 accordingto the embodiment is formed. Reference numerals 71 and 73 denotediffusion barrier films that also function as adhesion films.

Next, a description is given of a first variation of the embodiment ofthe present invention. The first variation is different from theabove-described embodiment only in that the single-crystal substrate onwhich a ferroelectric film is formed is a layered structure.

FIG. 7 is a sectional view of a layered structure 80 in which aferroelectric film 83 is formed according to the first variation of theembodiment. Referring to FIG. 7, a buffer layer 82 and the ferroelectricfilm 83 are successively stacked on a silicon single-crystal substrate81 whose main surface is the (100) or (010) surface, thereby forming thelayered structure 80. The buffer layer 82 is epitaxially grown on thesilicon single-crystal substrate 81, and the ferroelectric film 83 isepitaxially grown on the buffer layer 82.

The buffer layer 82 is formed of a MgO layer, an -yttrium-stabilizedZrO₂ layer (YSZ), a SrTiO₃ layer, a magnesia spinel (MgAl₂O₄) layer, ora CaO layer. When the buffer layer 82 is formed on the (100) surface ofthe silicon single-crystal substrate 81, the (100) surface of the bufferlayer 82 grows. When the buffer layer 82 is formed on the (010) surfaceof the silicon single-crystal substrate 81, the (010) surface of thebuffer layer 82 grows. The buffer layer 82 is grown to have a thicknessof 30-800 nm by MBE, CVD, or spattering. Specifically, in the case offorming a MgAl₂O₄ layer by CVD, for instance, the composition elementsof the MgAl₂O₄ layer are heated and vaporized in their respective sourcechambers, and are supplied into a film-forming chamber by a carrier gas.The single-crystal substrate 81 is heated to 750-1050° C. The depositionrate is set to 5-30 nm/min, and the MgAl₂O₄ layer is formed to have athickness of 80-600 nm.

The ferroelectric film 83 can be formed on the buffer layer 82 by MOCVD,MBE, PLD, or the sol-gel method described above in the embodiment of thepresent invention. The details of the formation method are the same, anda description thereof is omitted. When the ferroelectric film 83 isformed on the (100) surface of the buffer layer 82, the (100) surface ofthe ferroelectric film 83 grows epitaxially. When the ferroelectric film83 is formed on the (010) surface of the buffer layer 82, the (010)surface of the ferroelectric film 83 grows epitaxially. Accordingly, ineach case, the [001] direction, which is the direction of thepolarization axis of the ferroelectric film 83, is parallel to thesingle-crystal substrate 81. In order to form a ferroelectric capacitor,the surface orientation of the ferroelectric film. 83 may be specifiedso as to expose (001) surfaces thereof, and electrodes may be formed onthe exposed (001) surfaces.

According to the first variation, a silicon substrate is employed as thesingle-crystal substrate 81. Therefore, a large-size single-crystalsubstrate of 300 mm in diameter is available at a low price. As aresult, the semiconductor device can be manufactured at reduced costs.

A silicon single-crystal substrate having an inclination with an offsetangle (a vicinal substrate) may be employed. In the case of a siliconsingle-crystal substrate with no offset angle, upward and downward stepsdue to minute unevenness may exist when viewing a section of thesubstrate taken in one direction. When a buffer layer is epitaxiallygrown on the surface of such a substrate, the atomic layers are formedsideways on the upward and downward steps so as to collide with eachother, thus generating a grain boundary. In the case of employing avicinal substrate, however, no such phenomenon occurs. Accordingly,crystals of better quality may be obtained. This is also the case withthe crystallinity (crystal quality) of the ferroelectric film 83 formedon the buffer layer 82. The better the crystallinity of the buffer layer82, the better the crystallinity of the ferroelectric film 83. In termsof crystallinity, the offset angle is preferably in the range of 0.8-4degrees.

Next, a description is given of a second variation of the embodiment ofthe present invention. The second variation is different from the firstvariation only in that another buffer layer 85 is additionally formedbetween the buffer layer 82 and the ferroelectric film 83.

FIG. 8 is a sectional view of a layered structure 84 in which theferroelectric film 83 is formed according to the second variation. InFIG. 8, the same elements as those previously described are referred toby the same numerals, and a description thereof is omitted.

Referring to FIG. 8, the buffer layer 82, the buffer layer 85, and theferroelectric film 83 are successively stacked on the siliconsingle-crystal substrate 81 whose main surface is the (100) or (010)surface, thereby forming the layered structure 84. The buffer layer 82is epitaxially grown on the silicon single-crystal substrate 81, thebuffer layer 85 is epitaxially grown on the buffer layer 82, and theferroelectric film 83 is epitaxially grown on the buffer layer 85.

As the buffer layer 85, a SrRuO₃, YBa₂Cu₃O_(7-δ) (YBCO), orLa_(2-x)Sr_(x)CuO₄ (LSCO) layer of 60-240 nm in thickness is employable.The buffer layer 85 is formable in the same manner as the buffer layer82. The formation of the buffer layer 85 reduces the difference inlattice constant between the buffer layer 82 and the ferroelectric film83. As a result, the ferroelectric film 83 may be formed with bettercrystallinity.

Next, a description is given of a third variation of the embodiment ofthe present invention. The third variation is different from theabove-described embodiment only in that the ferroelectric capacitor 26is replaced with a ferroelectric capacitor 86 formed of a plurality offerroelectric capacitors connected in parallel.

FIG. 9A is a plan view of the ferroelectric capacitor 86 according tothe third variation. FIG. 9B is a sectional view of the ferroelectriccapacitor 86 of FIG. 9A taken along the line X-X.

Referring to FIG. 9A, ferroelectric films 87 are sandwiched betweenelectrodes 88A and 88B on the single-crystal substrate 21 so that fourparallel-plate ferroelectric capacitors are connected in parallel toform the ferroelectric capacitor 86.

The ferroelectric films 87 and the electrodes 88A and 88B are formableby changing the patterning of the resists 23 and 24 in the processes ofFIGS. 6A and 6B. The other processes are the same as in theabove-described embodiment.

According to the third variation, the ferroelectric capacitor 86 isformed of ferroelectric capacitors connected in parallel. Accordingly,the total remanent polarization can be increased so that the same chargecan be obtained with a reduced operating voltage. Simultaneously, asemiconductor device with reduced power consumption and high reliabilitycan be formed.

Thus, according to the present invention, in a capacitor, electrodes areprovided perpendicular to the direction of the polarization axis of aferroelectric film sandwiched between the electrodes. Accordingly, agreater remanent polarization charge may be obtained.

The electrodes may be plates and provided parallel to each other so asto ensure that the charge of the maximum remanent polarization of theferroelectric film is obtained.

A semiconductor device including such a capacitor has tolerance for thedegradation of the remanent polarization charge and has a highsignal-to-noise ratio at the time of reading out information.Accordingly, the reliability of the semiconductor device can beincreased.

Further, according to the present invention, the ferroelectric film maybe an epitaxial film. Generally, the epitaxial film is formed on asingle-crystal substrate with the same orientation relationship as thecrystals of the substrate. Accordingly, the epitaxial film has not onlycrystalline orientation in the direction in which it grows but alsocrystalline orientation of an in-plane direction.

Further, according to the present invention, since the ferroelectricfilm of the capacitor is formed on a single-crystal substrate, theferroelectric film may be formed epitaxially thereon. Therefore, theferroelectric film can be formed with excellent crystalline quality witha fixed crystal orientation. Further, in the semiconductor device, thecapacitor is formed in a process independent of a process of forming thetransistor. Accordingly, a wide range of tolerance can be set for theconditions for forming the capacitor, such as a temperature condition,so that the ferroelectric film may be formed with better crystallinity.As a result, the reliability of the semiconductor device can beincreased.

Further, according to the present invention, a buffer layer may beformed between a silicon single-crystal substrate and the. ferroelectricfilm. The formation of the buffer layer reduces the lattice mismatchingbetween the ferroelectric film and the silicon substrate. As a result,the ferroelectric film can be grown epitaxially with bettercrystallinity.

The present invention is not limited to the specifically disclosedembodiment, and variations and modifications may be made withoutdeparting from the scope of the present invention. For instance, each ofthe first and second variations may be combined with the thirdvariation.

1. A semiconductor device, comprising: a semiconductor substrate; atransistor formed on the semiconductor substrate, the transistorincluding a gate electrode and a diffusion region; a first interlayerinsulating film covering the semiconductor substrate and the transistor;a second interlayer insulating film formed on the first interlayerinsulating film; and a capacitor formed in the second interlayerinsulating film, the capacitor including a pair of electrodes and aferroelectric film sandwiched therebetween, wherein the electrodes areprovided perpendicular to a direction of a polarization axis of theferroelectric film.
 2. The semiconductor device as claimed in claim 1,wherein the direction of the polarization axis of the ferroelectric filmis substantially parallel to a main surface of the semiconductorsubstrate.
 3. The semiconductor device as claimed in claim 1, whereinthe ferroelectric film is an epitaxial film.
 4. The semiconductor deviceas claimed in claim 1, wherein the ferroelectric film comprises aperovskite structure, a bismuth layer structure, and a tungsten bronzestructure.